1. Field of the Invention
This invention relates generally to aircraft terrain clearance systems and, more particularly, to a terrain clearance system particularly usable with a ground proximity warning system for providing a terrain clearance signal when the radio altimeter signal is unreliable or not valid.
2. Description of the Prior Art
Various terrain clearance systems are known including radio or radar altimeters. While such radio or radar altimeters generally do provide an accurate reading of terrain clearance, there are conditions where they do not. Among these conditions are a malfunction in the altimeter or an out of track condition where the aircraft ascends above the maximum operating altitude of the radio altimeter, usually 2500 or 5000 feet, and does not recover from the out of track condition when the aircraft descends again. Other inaccurate readings can occur during aircraft maneuvering during extreme pitch or roll angles where the radio altimeter signal is not being directed vertically toward the ground, but diagonally toward the terrain or even above the horizon. Under such conditions the radio altimeter reading will be higher than the actual terrain clearance and give the pilot a false sense of security when the aircraft is actually dangerously close to terrain.
To overcome some of these problems, attempts have been made to utilize a combination of radio altitude and barometric altitude signals in various proportions as a function of the altitude so that the barometric altitude signal predominates at higher altitudes and, the radio altitude signal predominates at lower altitudes. Other systems utilize the last valid radio altimeter signal to provide an altitude datum and integrate barometric altitude rate to obtain a change in altitude relative to that datum. Examples of such a systems are disclosed in U.S. Pat. No. 4,987,413 to Grove and in U.S. patent application Ser. No. 07/082,562 filed by Grove on Jul. 31, 1987. A system that discloses integrating barometric altitude to provide an altitude change is disclosed in U.S. Pat. No. 4,433,323 to Grove.
While such systems do provide a way of estimating terrain clearance when the radio altitude signal is unreliable, it is difficult to determine when the actual radio altitude signal should be utilized and how to determine that the radio altitude signal is unreliable and utilize the estimated or pseudo-altitude signal instead. It is also difficult to determine whether the pseudo altitude is valid and how long the pseudo altitude should remain engaged before its accuracy becomes questionable.